I also bought a phone from an ebay seller and did not realize it was locked to Sprint. I have tried unlock software off the internet from 2 different companies claiming they can unlock and iphone 5 - both couldn't. I have called Sprint twice and they have said unless I live in USA I cannot unlock it through them, and Apple have said they can do nothing. I am in the same situation right now. I will most likely have to sell it back although the seller did say at one point he/she would replace the phone with an unlocked one but now doesn't seem so helpful. It is very frustrating but I guess the bottom line is that I should have read the fine print 🙂. Another lesson learned.
My question is "Why are phones being locked to a carrier at all?" Apparently companies can't lock to their own carriers here in New Zealand. It's not a scheme to help the customer, only the company its locked to.